#176 more control over when WW errors and messages are reported

[Lee Worden]

It would be better not to attach these things in parser hooks, because the parser is used in lots of auxiliary stuff like rendering internationalized messages, so the WW messages can sometimes show up in weird places - for instance, in the projects box in the sidebar. I would like to move that out of the parser hooks and only output the WW messages in a small number of well-defined points in the page.